1. Title of the Invention
The present invention relates to a bit rate reduction apparatus for reducing the data amount of video information.
2. Description of the Prior Art
To reduce the data amount when transmitting or recording video information, bit rate reduction is often employed. Bit rate reduction is a means for compressing the data amount by removing the redundant portion of the video information. In a conventional example of bit rate reduction, the input sample value is first divided into blocks of adjacent plural sample values, and each block is orthogonally transformed to be coded in each orthogonal transform. In the bit rate reduction making use of such transformations, the transformed component is quantized, and is transmitted by variable length coding.
The variable length coding is a method of coding a signal having a high frequency of occurrence into a code word of a small number of bits, and coding a signal of low frequency of occurrence into a code word of a large number of bits. As a result, data can be transmitted by a smaller number of bits in average. Therefore, in the conventional apparatus for recording or transmitting video information, the data is once curtailed by variable length coding, and then recorded or reproduced.
In such variable length coding, usually, the data amount after variable length coding may differ depending on the input video information. Accordingly, to keep constant the data amount after variable length coding, the data amount is controlled by a process called quantization prior to variable length coding. Quantization is a process of dividing an input transform component by a certain value to transform the same into a smaller value. This divisor is called the quantizing width. When the quantizing width is small, the data is transformed into a lager value by quantization (the compression rate is small), or when the quantizing width is large, the data is transformed into a smaller value by quantization (the compression rate is large). Therefore, by changing the quantizing width adaptively depending on the input data, the data amount after variable length coding may be kept constant.
Such an adaptive quantizing means is disclosed in U.S. patent application Ser. No. 07/741047, "Orthogonal transform coding apparatus", filed Aug. 6, 1991. In this patent, first, transformed components in each block after orthogonal transform are classified into plural sets. Then, in every classified set, one quantizing width is selected from plural quantizing widths, and quantization is executed. By thus selecting the quantizing width in each set, a combination of multiple quantization may be possible in the block units if the variety of quantizing widths is small. Therefore, minute data amount control is possible in a small circuit (few quantizing widths).